The INS Arihant, India's first nuclear-powered submarine armed with ballistic missiles (SSBN, in military jargon). The 6,000-tonne boat will provide India with the third leg of its nuclear "triad" - it already has land- and air-launched nukes...
India believes SSBNS are a vital part of its nuclear strategy, which forswears the first use of nuclear weapons. The Indian navy's latest statement of maritime strategy, published in October, says the country's nuclear-deterrence doctrine involves having a "credible minimum deterrent" that can deliver "massive nuclear retaliation designed to inflict unacceptable damage" in response to a nuclear strike against India. Because they can readily avoid detection, SSBNS can survive a surprise attack and thus ensure India's ability to launch a retaliatory "second strike".
Some nuclear theorists argue that submarine-based deterrents promote peace by making the other side more frightened to aatack first ("What lurks beneath. Asian nuclear weapons. A nuclear arms race at sea", THE ECONOMIST, February 6th 2016).